1. Field of the Invention
The present invention relates to a hand-held electrical combination hammer drill capable of being used both as a drilling hammer and as a chisel hammer, and including a housing having a handle, an actuation member for actuating an electrical on-off switch of the hammer drill and located on the handle, a switching element for switching between drilling and chiseling operations, and a stop element actuated by the switching element for retaining the actuation member in an on-position of the on-off switch upon selection of the chiseling operation.
2. Description of the Prior Art
Hammer drills of the type described above, as a rule, has an actuation member provided on the housing handle for actuating an electrical on-off switch and which often simultaneously serves as an adjusting element, e.g., for a potentiometer for, in particular, for effecting a stepless regulation of the electric motor speed, or which is connected with such an adjusting element. When the actuation member is depressed by the user, the motor drive of the hammer drill is turned-on, upon release of the actuation member, the motor drive is turned off.
In many hand-held electrical tools, such as drills, hammer drills, orbital sanders, sabre saws, there is provided a stop knob that locks the actuation member in its on-position. This knob provides for a continuous operation of the tool, so that the actuation member need not be continuously depressed. The stop knob releases the actuation member upon application of a stronger pressure to the actuation member, which is applied when the tool need be turned-off.
Contrary to the conventional electric tools, with hammer drills, for safety reasons, it is necessary or desirable to retain the actuation member in its on-position only during the chiseling operation, without a possibility of retaining the actuation member in the on-position during the drilling operation when the actuation member is not depressed. In the electrical combination hammer drill disclosed in German Publication DE 19720947A1, this problem is solved by providing, on a rotatable switch knob used for manual switching between the chiseling and drilling operations, a cam which rotates a pivot lever biased against the cam. Upon pivotal movement of the pivot lever, a lock cavity, which is formed thereon, extends into displacement path of a retaining member provided at the free end of a leaf spring secured on a locking pawl for an on-off switch. With this solution, the locking pawl, i.e., the actuation member for the on-off switch, upon actuation of the chiseling operation, becomes positively locked. With the switch knob pivoted by 180°, i.e., during the drilling operation, the cam-actuated pivot lever is pivoted out of the displacement path of the retaining member on the locking pawl, and locking of the locking pawl by the user is not any more possible.
The known solution has encounted numerous practical problems and difficulties. For one, the elements providing for locking of the actuation member during the chiseling operation, the locking cavity, the pivot lever, and the locking spring undergo a different degree of wear because of abrasion, which is caused by pressure applied by engaging each other elements to each other, and due to the fatigue phenomenon of the locking spring. In addition, upon selection of the chiseling operation, after actuation of the locking pawl, the continuous operation is always positively established. Therefore, the user must first release the locking pawl with his hand, if chiseling operation is to be interrupted, in order to place the locking pawl beneath its pivot point. However, there exists a number of chiseling processes when a continuous operation of the chisel is not desirable. E.g., a so-called scabble process when a plurality of discrete short, following one another, chiseling steps are executed. With this process, after removal of the sheeting or the form work, edge overhangs of a concrete construction are removed with an electrical chisel. During such operation, a continuous operation of the tool is not needed and is not desirable.
Accordingly, an object of the present invention is to provide a combination drilling/chiseling hammer drill in which the locking of the switch actuation member does not take place during the drilling operation, however, the chiseling operation can be effected continuously for a long period of time or with interruptions.